"This may have a particular impact on ovarian cancer, which is often diagnosed at a late stage.

"Understanding the genetic mistakes that drive these cancers may also lead to new ways to treat these diseases."

The researchers speculate that they have uncovered a new cancer-causing process, since the PPM1D gene appears to operate differently to other genes known to increase the risk of breast and ovarian cancer, such as BRCA1 and BRCA2.

The team found that changes in PPM1D were not inherited and rather than being present in every cell - as in most inherited cancer-causing genes - they were only present in immune cells known as lymphocytes.

Even more surprisingly, the PPM1D gene was not altered in women's cancer cells, nor in their normal breast or ovarian cells, the study discovered.

The changes make the PPM1D gene overactive, reducing the action of a gene called TP53, one of the most frequently altered genes in cancer cells.

The ICR's Professor Nazneen Rahman, who led the study, said: "This is one of our most interesting and exciting discoveries. At every stage the results were different from the accepted theories.

"We don't yet know exactly how PPM1D mutations are linked to breast and ovarian cancer, but this finding is stimulating radical new thoughts about the way genes and cancer can be related."

Professor Rahman said the findings could also help inform women's decisions about future medical treatments.

A woman might, for example, consider keyhole surgery to remove her ovaries after completing her family if she knew she had PPM1D changes and had a one in five chance of developing ovarian cancer.

The study was funded by Cancer Research UK, the ICR, the Wellcome Trust and Breakthrough Breast Cancer.

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